Different types of development attract different numbers of people.

Urban For each 1bp spent on a Building(⁺) in a Town or City - you attract about 100 people (*) to live in the town’s hex. Perhaps only a quarter of them live in the city itself – the rest live in suburban villages, hamlets and thorps within a few miles of the city walls. They provide many of the resources (Food, firewood, Labour etc) that you need to keep your town ticking over.

Rural In the countryside people want to be spread out more - each rural development or building (⁺) you develop attracts about 50 people (*) to the hex. It doesn't matter if it is a Watchtower, Keep, Tavern, Road House or Inn – it still only attracts about 50 people to the hex.

Wilderness This is a bit more complicated - But you attract about 10 people (*) for every BP spent on a Defensive building (Watchtower, Camp, redoubt) and about 10 people for every other building in the hex.

(⁺) Except Civic or Religious buildings.
(*) These are average numbers - and could vary by as much as 10% each way.